The present invention relates to a method and an apparatus for safeguarding an a.c. circuit, wherein the effective voltage applied to the load circuit is gradually raised, in particular, by enlarging the angle of the current flow, and wherein the load circuit is cut off when a predetermined load current is exceeded.
A process for reducing the surge of switch-on current when operating an inductive load, wherein the effective voltage at the load is increased by enlarging the phase operating angles of the voltage halfwave while measuring the current, is known in German Patent document DE A1 40 11 170. If a specific current threshold value is exceeded in the process, it is assumed that magnetization of the inductive load is at the end of the hysteresis curve. From this point on, switch-on occurs with a counterpoled voltage of a larger effective value.
When measuring the current during switch-on, it has always been assumed that when the current threshold value is exceeded, magnetization is at saturation, i.e. at the end of the hysteresis curve. However, no differentiation is made between active current and reactive current, so that it is not possible to determine if there is an overload or a short circuit in the a.c. circuit. An overload or short circuit causes a gradual increase in the current flow which exceeds the current threshold value and causes a switch-on. This subsequently results in tripping a safeguard element.